1. Field of the Invention
The present invention relates to an electronic device that can receive an AM radio broadcast signal and has a switching power supply circuit (including a chopper regulator).
2. Description of the Related Art
In general, an electronic device such as an audio system in which a tuner, a CD or MD player, a cassette tape recorder and the like are integrated with each other can receive an AM radio broadcast signal and uses a switching power supply circuit (including a chopper regulator) as a power supply.
In such an electronic device in the past that can receive an AM radio broadcast signal and has a switching power supply circuit (including a chopper regulator), high switching noise is caused by a switching signal of the switching power supply circuit. It is therefore necessary to separate a power supply unit and a tuner unit from each other within the electronic device, or to provide a shield by for example enclosing either the power supply unit or the tuner unit with a sheet metal or the like.
In particular, at a time of a sudden increase in the load of the electronic device, for example at a time of recording an AM radio broadcast signal on a cassette tape recorder or at a time of loading a CD into a CD player, an inconvenience is caused in that the switching signal of the switching power supply circuit is heard as a sudden noise even when an AM radio broadcast signal is received in good conditions.
Japanese Patent No. 3439049 discloses a related-art method of changing the value of a voltage (a low voltage or a high voltage) supplied to an amplifier by a switching circuit according to the magnitude of an input signal in an audio system in which a tuner, an amplifier, a CD or MD player, a cassette tape recorder and the like are integrated with each other. In this case, when the tuner operates near the amplifier, noise caused by switching of the switching circuit leaks into a radio broadcast, and is output as noise from the tuner. A harmonic component of a switching signal of the switching circuit performing high-speed operation, in particular, tends to be superimposed as noise in a frequency band of an AM radio broadcast signal. In order to eliminate this phenomenon, when an AM radio broadcast signal is received, the switching circuit is set in a non-operating state, and only a low voltage is supplied to the amplifier.